The present invention relates to moving filter devices, particularly moving filters designed to be used in air delivery fans.
Particulate air filters are conventionally formed of porous media. The particle laden air is passed through the porous media which removes the particulate based on physical entrapment, impaction, surface attraction, inertial forces or the like. The porous filter media can be porous films, open celled foams, woven fabrics, molded particles, or nonwoven fabrics or webs and the like. The filter media can be flat or formed into a three dimensional configuration (generally a pleated form). Pass through type filters will act on the entire airstream passed through the filter media with an associated pressure drop and filtration efficiency that is characteristic of the media, its level of particulate loading and the airstream velocity and pressure. Generally, as the filter media becomes loaded with particulates the pressure drop increases, however, the filtration efficiency can increase or decrease depending on the nature of the media and the particulates being removed.
Generally, most filters when used are static with the particle laden air driven through the filter. However, filters that move have been proposed, for example, to keep fresh filter media in the path of the airstream to be filtered as disclosed in U.S. Pat. No. 5,560,835 (driven slowly by drive rotor) or U.S. Pat. Nos. 4,038,058 and 3,898,066 (filter media driven by air impinging on rotor blades). These filters operate like conventional flow-through static filters and have the associated problem of pressure drop buildup over time. Flow through type filters have also been associated with faster moving devices such as rotating disk drives (U.S. Pat. No. 4,308,041), on an air inlet to a combine vent fan (U.S. Pat. No. 3,392,512), between fan blades on an air inlet fan for a turbine engine (U.S. Pat. No. 3,402,881), on a fume exhaust fan (U.S. Pat. No. 4,450,756), or in an air inlet to a building ventilation fan (U.S. Pat. No. 3,126,263). The proposed filters placed on a fan designed to circulate air (e.g., U.S. Pat. Nos. 3,402,881 and 4,450,756) have the filter media strategically placed to ensure that all the air passing through the system is passed through the filter media. In U.S. Pat. No. 3,402,881, the filter media 100 is woven between fan vanes 98 and sealed to prevent air from bypassing the filter media. This requires that the filter media be periodically cleaned. This cleaning is done by a complicated periodic backflow of air from the engine compressor or like source of high pressure air in the system. With U.S. Pat. No. 4,450,756, the filter must be periodically removed and cleaned or replaced. If the filter is not replaced when loaded, the pressure drop across the filter rises often to unacceptable levels, cutting off airflow. Although not desirable generally in certain filter applications this reduction in airflow is unacceptable. In automotive cabin applications, increases in pressure, due to filter media particle loading, can drastically reduce airflow which can result in dangerous window fogging.
In automotive or furnace filter applications, the general approach has been to place a filter at some location in the airstream (e.g., in the ducts) to intersect the entire airstream. Commercially the almost universal approach has been to place filters at various locations between the air inlet and air outlet in a vehicle or home heating and air conditioning system. An issue with these filters is they are often difficult to access unless they are located near the air inlet or outlet(s). However if the filter is located at or near an air inlet (which generally are easy to access) only incoming air or recirculated air is filtered, but not both, unless multiple filters are used at the air inlet(s) for fresh air and the air inlet(s) for recirculated air. In a novel variant of this general approach, U.S. Pat. No. 5,683,478 proposes placing a static filter inside a fan of the blower motor assembly intersecting the airstream immediately prior to the fan, as both recirculated and fresh air directed through the fan are filtered.
Generally, filter materials that are used function at very low pressure drops to ensure that the system, even if the filter is fully loaded with particulates, does not unacceptably reduce airflow. However, if the filter media is of the very low pressure drop type it generally is a low efficiency filter (e.g., an open nonwoven web), has a limited lifetime (e.g. charged webs with relatively low basis weight) or is very bulky (e.g., a heavily pleated filter), which is undesirable where there is limited space such as in an automotive Heating, Ventilation or Air Conditioning (HVAC) system. Alternatively, it has been proposed that a certain portion of the airflow bypass the filter to ensure that pressure drop does not rise unacceptably during the lifetime of the filter. An air bypass of this type can eliminate the issue of unacceptably reduced airflow through the HVAC system due to a fully loaded filter but severely impacts filtration efficiency, particularly when filtering incoming air. Ideally, to ensure adequate airflow to an automotive cabin, the pressure drops of a filter in the HVAC system should show little or no pressure drop over its lifetime, no matter how long it is in use. Similarily, home heating system filters should not significantly reduce airflow even when fully particle laden.